Anomalous Azide Binding to Metmanganomyoglobin

The reaction of metmanganomyoglobin (Mn^IIIMb) with azide presents a novel pattern with direct evidence for kinetic complexity. Although the kinetic analysis may not be unique, it appears that the azide complex cannot be fully formed, as judged by spectrophotometric changes, even with an infinitely great [N₃^-]. This is interpreted as resulting from an equilibrium between the final spectroscopically observable azide complex and an intermediate species whose spectrum is not substantially different from that of Mn^IIIMb itself. The two forms of the azide complex appear to exhibit roughly equal proportions at 3 °C. We propose that this intermediate is a weak Mn³⁺-azide complex in which the metal ion remains out-of-plane toward the imidazole of the proximal histidine, but that the metal lies toward the anion in the “final” complex.